The present invention relates to an anti-aliasing filter circuit for digital oscilloscopes and particularly to such a circuit which operates in conjunction with the oscilloscope's sweep-rate setting.
Modern digital storage oscilloscopes sample an input signal at a multiplicity of discrete points, and store digital values representative of these points in random access memory. The memory is then read out for portrayal of the stored waveform on the oscilloscope's cathode-ray-tube.
Whatever the sampling rate, it is possible for some of the original analog information to be lost. Moreover, the sampling rate is, in fact, somewhat limited by the maximum storage available in the oscilloscope. The samples may be too far apart, timewise, particularly at slow sweep speeds, for an accurate representation of waveform information to be given across the face of the oscilloscope's cathode-ray-tube. Thus, if the frequency of the input signal information is greater than the Nyquist rate, equal to half the sampling frequency, a waveform presentation may be produced which is very non-representative of the actual input signal. The observed distortion is known as "aliasing".
Aliasing can be avoided by limiting the bandwidth of input so that frequencies greater than half the sampling frequency are filtered out. However, this expedient may limit the frequency range over which the oscilloscope is effective or requires an excessive number of analog filtering devices.